The document U.S. Pat. No. 6,343,767 already discloses such an optical viewing device, with a mobile orientation, to observe a field through a window being substantially dome-shaped. The field is projected on a viewing plane through the viewing device, such device comprising to this end optical conjugation means for the field with the viewing plane and relative orientation means for the viewing plane with respect to said field being observed.
More precisely, said relative orientation means are made of mobile prisms being rotated around the optical axis of the device. Thus, they allow the viewing direction to be oriented in a cone, the half-angle of which at the summit is equal to 2·(n−1)·A, A being the angle at the summit of the prisms and n the refraction index for the material of said prisms.
Such a device, also called “diasporameter”, thus allows a field centred on the optical axis of said device to be observed, such viewing field being able to be reoriented with respect to said optical axis via the set of prisms so as to view a different optical field and located close to the field being centred on said optical axis. This device thus offers the possibility, via the rotating prisms, to observe a very broad optical field as well as to restore the viewing when the latter is deviated from the optical field looked at, being centred on the optical axis.
Such a device is generally used for an infinity observation and works in such a case with parallels beams. Under these conditions, the prisms are not able to degrade the optical quality of the image formed on the viewing plane, except possibly for the chromatic and dispersal aberrations.
Nevertheless, such a device has a major disadvantage bound to applications such as auto-leading guidance of a missile. Indeed, for such applications, it is indispensable that the window being located upstream from the missile path presents an aerodynamical profile and more particularly a substantially hemispherical dome shape.
The shape of such window then introduces a non nil divergence corresponding to a focal distance, for example, of −20 to −5 meters, thereby necessarily generating aberration defects, especially of the astigmatism type. In other words, due to the curvature of the window, the field focusing steps in two fields being perpendicular to each other cannot be performed each in a same viewing plane perpendicular to the optical axis of the device. The image being formed on such viewing plane as well as the viewing performance then become substantially degraded.
Furthermore, the above-mentioned relative orientation means, such as prisms, are designed for parallel beams. Now, in the present case, the beam that crossed the window is substantially divergent. Consequently, the defects generated at the window will reach the prisms and be amplified therein.
Consequently, the assembly formed by the window and the relative orientation means is the source of significant aberration defects, especially of the astigmatism type.